So you have Old Gods and Legacy of Rome. Those are the most important two, though Way of Life and Conclave (and maybe Reaper's due? I bought it instantly) will improve your game somewhat regardless of who you play.
Legacy of Rome opens a lot of cool Byzantine stuff, but also unlocks retinues which are vital... Once you're a king. As a duke or count you don't really need them.
Charlemagne provides some unusual options (I own a completely absurd amount of territory, I hereby declare it a kingdom) and unlocks an earlier start date, but otherwise isn't that important.
The thing to be careful about, is Old Gods. Playing as a pagan (probably Norse) is... in a way, like playing any other strategy game. You don't need a reason to conquer. That might detract from your learning experience.
Same with Republics, these are "advanced" (nuanced yet much EASIER) modes.
The way I learned was to play an Irish count in 1066. After several generations of struggle, I united Ireland - then quit.
Personally, I think that's the way to learn the game. And here's what you need to know:
Advisors should always be doing something.
Unless you're a vassal, your counselor should be fabricating a claim.
Improving your holdings can wait until you have two duchies, maybe one. It's a *very* long term investment.
Borrow from the Jews and don't pay it back until you're about to quit.
Marriage is important. Always marry your daughters matrilinearly.
...
Hold fairs and stuff. Have fun.
Play your traits. If you're cruel, maybe *do* torture that guy, even though there's no reason to.
Your dynasty is more important than yourself. But score doesn't matter.
Fuck the Karlings.
...literally, if possible, but MATRILINEARLY